The present invention is directed to systems, methods, and apparatus for calibrating or verifying laser ablation systems. In particular, embodiments relate to methods and apparatus for measuring the refractive power, shape and quality of a laser test ablation on a test surface through interferometric techniques. Embodiments of the present invention are particularly useful for calibrating excimer lasers used during laser ablation procedures of the eye, such as photorefractive keratotomy (PRK), phototherapeutic keratectomy (PTK), laser in situ keratomileusis (LASIK), or the like.
Ultraviolet and infrared laser based systems and methods are known for enabling opthalmological surgery in order to correct vision defects and other precision structure modifications. These procedures generally employ an ultraviolet or infrared laser to remove a microscopic layer of an anterior stromal tissue from the cornea to alter its refractive power. In ultraviolet laser ablation procedures, the radiation ablates corneal tissue in a photodecomposition that does not cause thermal damage to adjacent and underlying tissue. Molecules at the irradiated surface are broken into smaller volatile fragments without substantially heating the remaining substrate; the mechanism of the ablation is photochemical, i.e. the direct breaking of intermolecular bonds. The ablation penetrates into the stroma of the cornea to change its contour for various purposes, such as correcting myopia, hyperopia, and astigmatism.
In such laser based systems and methods, the irradiated flux density and exposure of the cornea to the laser radiation are controlled so as to provide a surface sculpting of the cornea to achieve a desired ultimate surface change in the cornea. To that end, ablation algorithms have been developed that determine the approximate energy density that can be applied to remove a certain depth of tissue from the cornea. At ultraviolet wavelengths, for example, a cumulative energy density of about 1 Joule/cm2 will typically ablate corneal tissue to a depth of about one micron when applied in a series of pulses of about 100 to 400 milliJoules/cm2. Accordingly, the ablation algorithms are tailored for each procedure depending on the amount and the shape of corneal tissue which will be removed to correct a particular individual's refractive error.
In order to properly use these laser ablation algorithms, the laser ablation system typically should be calibrated. Calibration of the laser system helps ensure removal of the intended shape and quantity of the corneal tissue so as to provide the desired shape and refractive power modification to the patient's cornea. In addition, it is usually desirable to test for acceptable levels of system performance. For example, such tests can help ensure that internal optics are aligned, that laser fluence is accurate, that desired beam characteristics are maintained, and the like.
When performing laser eye surgery such as when ablating a target region on a patient's cornea with a refractive laser beam system, it is beneficial to have accurate information on the energy, dimensions, and other characteristics of the laser beam spot which is incident on the cornea in order to determine the best achievable surgical outcome. For calibration purposes, it is particularly useful to have detailed information regarding the pulse energy in order to ensure that the correct profile is ablated onto the patient's cornea.
Ablations of plastic test materials are often performed prior to excimer laser surgery. During these tests, a lens is ablated into the test plastic, and the refractive power of the test lens is read by a standard lensometer. The reading from the lensometer is then entered back into the laser system so that the system can make appropriate calibration adjustments. The test lens may also be visually evaluated under a magnifying glass or with the microscope of the laser system, and test samples are sometimes sent to a laboratory for accurate evaluation to help determine beam homogeneity and quality.
Although known laser ablation calibration techniques are effective, some methods still suffer from certain disadvantages. For example, delaying each surgery while obtaining accurate laboratory evaluations of a test lens may be impractical. Nonetheless, some information beyond refractive power and a visual evaluation of the test lens would be helpful to improve the accuracy of regular calibrations, whether they are performed monthly, daily, or before each ablation procedure.
In light of the above, it would be desirable to provide improved systems, methods, and apparatus for calibrating laser ablation procedures. It would be particularly desirable if such improvements enhanced calibration accuracy without significantly increasing the overall system costs and complexity. It would further be desirable if such improvements could provide quantifiable data which might be used in an automated calibration feedback and adjustment system.